The following description relates to coupling electromagnetic energy into an opening smaller than the wavelength of the electromagnetic energy desired to be coupled.
Photons are expected to replace electrons in future all-optical devices as described in R. Biswas, M. M. Sigalas, K.-M. Ho and S.-Y. Lin, Physical Review B, Volume 65, page 205121, 20 May 2002.
Photonic crystals are optical devices that have become attractive due to their ability to control the propagation of light on a very small length scale. However, the small size of photonic crystals also poses one of the greatest challenges of employing them, such as finding an efficient way of coupling light into these ultra-small structures, as described in N. Moll and G.-L. Bona, Journal of Applied Physics, Volume 93, Number 9, page 4986, 1 May 2003. A fundamental constraint in manipulating light and coupling it into photonic crystals has to do with the extremely low transmission of light through apertures smaller than the wavelength of the incident photon. This limitation is known as the diffraction limit d=1.22λ/a, wherein “λ” is the wavelength of the light and “a” is a maximum dimension of the aperture.